Formatting consecutive numbers

Question

I\'m trying to format a list of integers with Python and I\'m having a few difficulties achieving what I\'d like.

Input is a sorted list of Integers:

Answer 1

Since the other guy who posted this solution deleted his answer...

Here's a O(n) string building solution:

def stringify(lst):
    result = str(lst[0])
    end = None
    for index, num in enumerate(lst[1:]):
        if num - 1 == lst[index]:  # the slice shifts the index by 1 for us
            end = str(num)
        else:
            if end:
                result += '-' + end
                end = None
            result += ', ' + str(num)
    # Catch the last term
    if end:
        result += '-' + str(num)
    return result

See the repl.it